1. Field of the Invention
The present invention relates to a planar lighting device for use as a backlight of a liquid crystal display panel.
2. Description of the Related Art
FIG. 18 shows a conventional planar lighting device 1 of this type for lighting a liquid crystal display panel (A) from behind. The planar lighting device 1 includes a light guide plate 2 having a pair of opposite light entry faces 2a, a light non-emitting face 2b and a light emitting face 2c, the light non-emitting face 2b being printed with plural dot patterns 2d. A light source 3 and a reflector 4 are disposed to extend along each of the light entry faces 2a. A reflecting plate 5 is disposed to extend along the light non-emitting face 2b. A diffuser 6, a first lens 7, a second lens 8 and a diffuser sheet 9 are disposed in this order to extend along the light emitting face 2c. 
When the light sources 3 of the planar lighting device 1 disposed at the back of the liquid crystal display panel (A) are turned ON, light from each light source 3 becomes incident on respective light entry face 2a either directly or via the reflector 4 and enters the light guide plate 2. Then, light is irregularly reflected by the dot patterns 2d and emitted from the light emitting face 2c. After light thus emitted is diffused by the diffuser 6, the direction of light is corrected through lenses 7 and 8, and then light is further diffused by the diffuser sheet 9. Finally, light is applied onto the liquid crystal display panel (A).
Such a prior art lighting device is adapted to reflect light irregularly by means of plural dot patterns 2d and hence involves a problem of a high loss of light due to scattering of light. For this reason, such a loss of light must be compensated for by enhancing the output power of light sources 3 or increasing the number of light sources 3 used, thus resulting in the device upsized as a whole with an increased cost.
Accordingly, it is a main object of the present invention to provide a planar lighting device which is capable of realizing a higher luminance without incurring inconveniences such as upsizing of the device.
According to the present invention, there is provided a planar lighting device comprising:
a light guide plate having a pair of opposite light entry faces, a light emitting face extending perpendicularly to the light entry faces, a plurality of hemispherical surfaces formed on the light emitting face, and a light non-emitting face opposite to the light emitting face;
a light source disposed to extend along each of the light entry faces;
a reflector for reflecting light from the light source toward a respective one of the light entry faces;
a reflecting plate disposed to extend along the light non-emitting face for reflecting light leaking from the light guide plate toward the light guide plate; and
a lens sheet disposed to extend along the light emitting face and formed with a plurality of first ridges extending parallel with the light entry faces on a face thereof opposite to the light emitting face, the first ridges each having a triangular section.
When the light sources 14 of the planar lighting device thus constructed according to the present invention are turned ON, light from the light sources enters the light guide plate from the light entry faces either directly or via respective reflectors. Light in the light guide plate passes through a wall surface of the light guide plate and is emitted to the outside when the angle of incidence on the wall surface meets a predetermined condition. The reflecting plate reflects light leaking from the light non-emitting face to return it into the light guide plate. Light emitted from the light emitting face becomes incident on the lens sheet. The direction of light incident on the lens sheet is corrected to a direction perpendicular to the light emitting face by means of the first ridges, and then light is applied to a liquid crystal display panel coupled with the lighting device.
Light reflected by the hemispherical surfaces of the light emitting face and light passing through the hemispherical surfaces are regularly directed so as to advance in a predetermined direction and, therefore, there is no possibility of scattering of light which would otherwise be caused by dot patterns provided in the prior art. Further, since light emitted from the hemispherical surfaces exhibits directionality with an inclination relative to the light emitting face at a predetermined angle, the direction of substantially the whole of light incident on the lens sheet is corrected to the direction perpendicular to the light emitting face by means of the first ridges.
Since the direction of light is corrected by both the hemispherical surfaces of the light emitting face and the first ridges of the lens sheet, the efficiency of light supply to the liquid crystal display panel can be improved by leaps and bounds.
Preferably, the planar lighting device further comprises a plurality of second ridges formed on the light non-emitting face of the light guide plate to extend in a direction perpendicular to the first ridges, the second ridges each having a triangular section, and a diffuser sheet disposed to extend along a light emitting face of the lens sheet.
With this feature, the direction of light is further corrected by the second ridges and, hence, the efficiency of light supply to the liquid crystal display panel can be further enhanced.
In a preferred embodiment of the present invention, the planar lighting device further comprises a reflection-type polarizing film which allows one polarized component of light from the lens sheet to pass therethrough but reflects other polarized component of the light, the reflection-type polarizing film being disposed to extend along a light emitting face of the lens sheet.
With this feature, only one of p-polarized light and s-polarized light is superposed on itself and applied onto the liquid crystal display panel and, hence, the luminance of the liquid crystal display panel can be further enhanced.
In another preferred embodiment of the present invention, the light emitting face of the light guide plate is formed with a plurality of projections each having an outward surface forming each of the hemispherical surfaces.
According to this feature, the outward surface of each projection forms each of the hemispherical surfaces.
In an alternative preferred embodiment of the present invention, the light emitting face of the light guide plate is formed with a plurality of depressions each having an internal surface forming each of the hemispherical surfaces.
According to this feature, the internal surface of each depression forms each of the hemispherical surfaces.
The foregoing and other objects, features and attendant advantages of the present invention will become apparent from the following detailed description of the present invention when read in conjunction with the accompanying drawings.